This invention relates to devices for making a no-contact measurement of the relative velocity, position, or diplacement path of objects without the use of special optical markers relative to a reference position in at least one coordinate direction, or of the degree of image contrast. The object is imaged on a grating and the light fluxes leaving this grating are measured by means of photoelectric means. The output signals of the photoelectric means exhibit a frequency component proportional to the velocity of travel.
Optical speed measuring devices are known wherein the relative movements of an object with respect to a photoelectric receiver produce measuring variables dependent on the relative velocity. In this connection, three different methods are basically employed:
In the first method, the object to be measured is irradiated with a monochromatic coherent radiation; the light scattered on the object is detected, the frequency of which is affected due to Doppler shift; and then the frequency of the scattered light is measured by interference with the primary radiation or with light freuquency-shifted in the opposite direction. This conventional procedure as disclosed in J. T. Ator: Image Velocity Sensing by Optical Correlation Applied Optics, August 1966 Vol. 5, Nr. 8, 1935 necessitates an expenditure in apparatus due to the required lasers and interferometers, which is in many cases intolerable.
With the second method, the object moved is reproduced in an image plane wherein at least two photoelectric scanners offset in the direction of movement are disposed. The signal of the first scanner, arranged as the first in the direction of motion, is stored, for a preselected time in a short-time storage means and thereafter compared, in a correlator, with a signal supplied by the subsequent, second scanner displaced at a distance along the path of movement. In this connection, the storage time .tau. is controlled so that the signal of the second scanner is chronologically congruent with the delayed signal of the first scanner. In this case, the velocity of the object image relative to the scanners, from which the object velocity v.sub.o is obtained via the imaging scale, results as: EQU v'.sub.o =c/.tau.
This conventional process as disclosed in F. Mesch, H.H. Daucher, R. Frische: Geschwindigkeitsmessung mit Korrelationsverfahren, Messtechnik 8, 1971 requires, with a controllable storage means and a correlator, a complicated, trouble-prone accumulation of apparatus and yields only an average velocity, obtained by averaging over the storage time .tau..
According to the third method, the object moved is reproduced on a grating having a number of k lines per mm. Behind this grating a photoreceiver receives the light coming from the object and preferably yields an alternating voltage upon the presence of a specific position frequency in the brightness distribution of the object. The frequency f of this alternating voltage is proportional to the velocity v of the object image relative to the grating and to k. The following applies: EQU f = v =.sup.. k
This signal is superimposed by longer-cycle signals (constant light) corresponding to the integral over the image portions with position frequencies which do not correspond to k. In order to suppress this constant light proportion, a device is known with a special pair of photoelectric receivers with telescoped, strip-shaped electrodes. This device supplies a push-pull signal only from image portions having a position frequency corresponding to k, and the in-phase portions of other signals are eliminated by a difference-forming bridge circuit. The special photoelectric receivers of this conventional device which is disclosed in German Published Patent Applicatiton 1,564,450 again require great expenditure, because of their difficult geometry and, since they cannot be produced at any desired fineness they limit the number of strips. The number of strips is directly connected to the measuring accuracy of the system.
It is known to measure movements of a real image of an object by introducing a grating into the image plane, collecting the light flux penetrating this grating in a photoelectric receiver, and determining the frequency of an A.C. component of the photoelectric current produced by the changing combined effect of the image with the grating structure. The possibilities for using such devices are very limited, due to the poor signal-to-noise ratio. One method of the type mentioned above and disclosed in British Patent No. 1,249,302 is able to overcome this deficiency by providing, as the grating, periodic arrangements of planar surfaces in a prismatic or pyramidal form, so that the light flux emanating from the object is collected in various directions by various photoelectric receivers. By this beam splitting method, it is possible to obtain, from the output signals of the photoelectric receivers, push-pull signals of an increased signal quality, from the frequency of which the travel velocity is determined. However, this method has the disadvantage that it yields signals which are ambiguous with regard to the arithmetic sign of the travel direction. However, the arithmetic sign of the movement is of importance in many practical cases, especially when it is intended to keep aiming devices directed to the measuring objects, by means of servo mechanisms.